(a) Field of the Invention
This invention relates to catalyst-supported particulate filters.
(b) Description of the Related Art
Exhaust gases from diesel engines using fuels containing diesel oil as a main component and exhaust gases from gasoline engines using fuels containing gasoline as a main component to burn them under fuel-lean conditions are known to contain particulates (suspended particulate matters containing carbon particles). To restrain particulates from being exhausted to the atmosphere, a filter for trapping particulates in exhaust gas is disposed in an exhaust gas passage of such an engine. When the amount of particulates deposited on the filter becomes large, this causes deterioration of engine power and fuel efficiency. Therefore, in this case, it is necessary to burn the deposited particulates as appropriate to remove them from the filter.
To effectively burn such deposited particulates (ignite the particulates at relatively low temperature and complete their burning in a short period of time), a catalyst layer containing catalytic metal-carried alumina is formed on the walls of exhaust gas channels in the body of such a filter. Catalytic metal-carried alumina is effective in burning particulates but there have been recently developed catalyst materials for filters capable of more efficiently burning particulates.
For example, Published Japanese Patent Application No. 2006-326573 describes the use of such a catalyst material in a filter. The catalyst material is a material in which a catalytic metal, such as platinum (Pt), is carried on a mixed oxide (composite oxide) containing cerium (Ce), zirconium (Zr) and a rare earth metal R selected from among praseodymium (Pr), neodymium (Nd) and lanthanum (La). The published document also describes that the content of R in the mixed oxide is preferably 2% to 11% by mole, both inclusive. Since such a mixed oxide contains Ce, it has oxygen storage/release capacity so that oxygen released from the mixed oxide contributes to ignition and burning of particulates.
Published Japanese Patent Application No. 2007-54713 describes the use of another catalyst material in a filter. The catalyst material is a material in which a catalytic metal is carried on mixed oxide ZrRO particles containing Zr and a rare earth metal R selected from among ytterbium (Yb), Nd and scandium (Sc) and having an R content of up to 18% by mole and mixed oxide CeMO particles containing Ce and a rare earth metal M selected from among samarium (Sm) and gadolinium (Gd). The ZrRO particles have oxygen ion conductivity to thereby release active oxygen but their oxygen release mechanism is different from that of CeZr-based mixed oxides as disclosed in Published Japanese Patent Application No. 2006-326573.
Specifically, CeZr-based mixed oxides have high oxygen storage capacity and release active oxygen by the valence change of Ce ions. On the other hand, the ZrRO particles have oxygen ion conductivity, i.e., so-called oxygen pumping function, and transport, when having both of high-oxygen concentration sites and low-oxygen concentration sites on the particle surfaces, oxygen ions from high-oxygen concentration sites to low-oxygen concentration sites and then release them as active oxygen.
Therefore, for the ZrRO particles, when a small ignition site causing particulates to burn is formed on their surface and then falls into lack of oxygen, oxygen is transported from other high-oxygen concentration sites. Thus, the burning is continued and the burning region on each particle can be easily extended from the ignition site to the surrounding sites.
Published Japanese Patent Application No. 2007-83224 describes the use of still another catalyst material in a filter. The catalyst material is a material in which a catalytic precious metal is carried on ZrRO having oxygen ion conductivity as stated above and alumina.